Add fixture coverage for Kotlin kotlinx-serialization

[schani]

Why

The Kotlin renderer's kotlinx-serialization framework (--framework kotlinx) had zero end-to-end fixture coverage — only klaxon (kotlin/schema-kotlin) and jackson (kotlin-jackson/schema-kotlin-jackson) were tested. Any change to KotlinXRenderer.ts shipped unvalidated. In particular, this unblocks #2845: once that PR rebases onto this, its kotlinx serializer changes get validated by CI.

What

• test/fixtures/kotlinx/ — driver mirroring the klaxon/jackson fixtures: main.kt round-trips JSON through the generated @Serializable types using kotlinx-serialization-json (with explicitNulls = false so nulls are omitted on encode, matching the other Kotlin fixtures' allowMissingNull behavior). The runtime jars (1.7.3) are vendored like the klaxon/jackson jars; the compiler plugin ships inside the Kotlin distribution (lib/kotlinx-serialization-compiler-plugin.jar), which build.sh locates relative to kotlinc.
• KotlinXLanguage in test/languages.ts, fixtures kotlinx / schema-kotlinx in test/fixtures.ts.
• CI: new kotlinx,schema-kotlinx matrix entry on the 16-core runner (the existing contains(matrix.fixture, 'kotlin') conditions install Java + Kotlin for it).

Covered vs skipped

Every JSON input (priority, samples, misc) and every schema was compiled and round-tripped locally with kotlinc 2.1.20 to build the skip lists; each skip is a verified, pre-existing kotlinx renderer limitation, not a guess:

• Unions (20 JSON inputs, 20 schemas): rendered as sealed classes with no serializer wiring — the documented TODO in KotlinXRenderer.ts; deserialization fails at runtime. The union feature is not claimed, so .fail.union.json samples don't run. feat(kotlin): Support for date and datetime from JSON schema #2845 fixes this; these skips can then be lifted.
• Top-level arrays (40 JSON inputs, union.schema): rendered as typealias TopLevel = JsonArray<T>, which doesn't compile (kotlinx's JsonArray takes no type arguments) — also a documented TODO.
• blns-object.json, identifiers.json: stringEscape emits astral-plane characters as 5-hex-digit \u escapes in @SerialName, which Kotlin misparses (4 digits only), so keys don't match.
• keyword-unions.schema: unions, plus the synthesized serializer constructor exceeds the JVM 255-parameter limit.

Everything else runs — 149 of 211 JSON inputs and 46 of 67 schemas, including inputs the klaxon fixtures skip: simple-identifiers.json, no-classes.json, bug427.json, and the any, uuid, keyword-enum, top-level-enum, top-level-primitive, nested-intersection-union, and go-schema-pattern-properties schemas.

Local verification

• FIXTURE=schema-kotlinx script/test: green (all 67 schemas, exit 0), with kotlinc 2.1.20 + Java 21.
• FIXTURE=kotlinx script/test (full, including misc): every input was verified individually via generate→compile→run→compare with the harness's comparison semantics; the full harness run was still in progress when this PR was opened, so CI is the final gate there. The two misc skipDiffViaSchema entries (34702.json, 76ae1.json) mirror the klaxon list; diff-via-schema for priority/samples was verified to match klaxon's behavior exactly (only bug427.json/keywords.json differ). Misc inputs don't run in CI (QUICKTEST=true).

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